Method to Extract Diffusion Length from Solar Cell Parameters—Application to Polycrystalline Silicon

TARETTO, K. R.; RAU, U.; WERNER, J.-H.

JOURNAL OF APPLIED PHYSICS

American Institute of Physics

Lugar: Nueva York; Año: 2003 vol. 93 p. 5447 - 5455

0021-8979

A closed form, analytical expression for the interdependence of the effective diffusion length Leff and the open-circuit voltage of solar cells is derived for the parallel connection of recombination in the space-charge region and in the neutral base region. This expression allows for the calculation of Leff from the open-circuit voltage, the short-circuit current, and the base doping of the solar cell as the only quantities that need to be determined experimentally. Values of Leff calculated with our method match with an accuracy of 35% values that are determined experimentally by quantum-efficiency measurements of silicon solar cells. The agreement holds in a range 0.3 µm < Leff < 300 µm. Alternatively, Leff is calculated from the short-circuit current and from the overall light absorption in the solar cell via an analytical expression for the Leff-dependence of carrier collection. Again, we find a good match between the measured Leff and the values calculated by this method. We further analyze photovoltaic output parameters from literature data of polycrystalline silicon solar cells covering grain sizes from 10–2 to 104 µm. We calculate Leff for these solar cells with our method and interpret the results in terms of grain-boundary recombination velocity SGB. We find that the data points split into two distinct groups, one with 105 cm/s < SGB < 107 cm/s, and one showing 101 cm/s < SGB < 10 3 cm/s. Since all cells with low SGB have a {220} surface texture, we ascribe the low-recombination velocity of grain boundaries to the low-electronic activity of [110]-tilt grain boundaries in these films.